High-intensity focused ultrasound (HIFU) therapy is a non-invasive and non-traumatic approach. It is particularly applied to the patients with tumors. Comparing to the conventional surgeries or chemotherapy, HIFU therapy brings the patient less trauma and therefore its applications have been developed rapidly. Its indications include liver cancer, bone sarcoma, breast cancer, pancreas cancer, kidney cancer, soft tissue tumor and pelvic tumor.
Most of the existing HIFU therapy use B-mode ultrasound imaging device to locate the region of the subject and to monitor the therapy. Adoption of B-mode ultrasound imaging device has the following advantages: low cost, real time imaging, having the same acoustic path as the therapeutic ultrasound, observing the tissue necrosis after high-intensity focused ultrasound (HIFU) exposures according to gray scale changes of the images. But, B-mode ultrasound image is only a plane image with a certain angle and cannot completely display the relative tissue relationships and solid structure of the region of the subject to be treated, therapeutic acoustic path and the area behind the region of the subject. Even though the 3-D ultrasound system is used, the visible area is still limited. Besides, the ultrasound image is limited on the depth of observation and it almost cannot display the tissue behind the bone because the bone influences the image greatly. The serious noises on the images exist during monitoring treatment. Further, ultrasound images have poor capacity to identify the tissue boundary and particularly it is more difficult to identify small tumors and deep-bedded tumors.
Nuclear Magnetic Resonance Imaging (NMRI) is an important application in biological and medical fields. It has a short name of MRI (Magnetic Resonance Imaging) and also refers to Nuclear Magnetic Resonance• CT (CT is short for computer tomography). The simple principle of MRI is: the patient lies inside an imaging magnet. Radio-frequency signals are then applied to the patient. The hydrogen nuclei in region of the subject are excited by radio-frequency signals and sends weak radio-frequency signals, which refer to nuclear magnetic resonance signals. During this process, the appropriate gradients are applied to the magnetic field so that the magnetic resonance signals can be acquired selectively. The information is processed to gain the tissue characteristics of each point and further the tissue can be imaged.
Magnetic Resonance Imaging (MRI) has great ability to identify different tissue and is easy to distinguish the normal tissue and tumor tissue and to determine the boundary of tumor tissue. MRI provides the volume data of a subject and a part of human body or full body can be imaged, therefore MRI is very suitable for locating the region of the subject to be treated by HIFU and planning HIFU surgical procedures. Meanwhile, with the development of MRI technology, the existing MRI equipment already can gain the images of the tissue in real-time, moreover the image is three-dimensional image with a certain volume. Therefore, MRI provides an excellent technical solution in monitoring the treatment procedures in real-time. Particularly, the temperature image provides a noninvasive temperature measuring method expected by the thermal therapy and has very important significance in controlling HIFU therapeutic dose, treating the region of the subject in time and controlling the energy.
In this art, it has been discovered that the ultrasound therapy for internal tissue of patient is monitored and guided by MRI. In HIFU surgery, MRI may be used to scan the patient for locating the region of the subject to be treated before HIFU treatment and also to guide the ultrasound wave to the region of the subject and monitor the temperature changes of the tissue during HIFU treatment so as to ensure that only the region of the subject is heated without destroying the surrounding normal tissue. The advantages of MRI are well known by the technicians skilled in this art.
At present, the major problem for a MRI guided ultrasound therapy apparatus focus is the interference between a MRI system and a focused ultrasound therapy system. Running of MRI system requires a steady strong magnetic field. In order to ensure the intensity and steadiness of the magnetic field, particularly for a relatively closed magnetic field, when the ultrasound transducer is operated in the magnetic field, the space for its mechanical motion locating means is limited greatly. In the field of this art, for this problem, some patents have already aimed to provide some solutions.
Japanese Patent No. 3322649 discloses a therapy system combined a MRI with an ultrasound therapy equipment. This system employs MRI to determine the location of a tumor firstly, and then the patient is moved out from the magnetic field of MRI and then treated by ultrasound. This kind of treatment needs repeated moving of patient and needs location for many times. The locating system is complex and a long time is needed to make locations. Furthermore, the real-time monitoring and on-line monitoring during the treatment are difficult to be realized.
U.S. Pat. No. 5,275,165 “Magnetic resonance guided ultrasound therapy system with inclined track to move transducers in a small vertical space” provides a MRI surgery system, which facilitates surgery with a focused ultrasound transducer that selectively destroys tissue in a region within a subject. The focused ultrasound transducer focalizes energy at a focal point within the region of tissue to be destroyed. A non-magnetic moving positioning device having a vertical dimension movement small enough to fit easily within the bore of a MRI magnet drives an ultrasound energy applicator in a limited vertical space. The positioning device employs a plurality of hydraulic positioners and an inclined plane to position the ultrasound focal point under the control of an operator. A MRI system employing a temperature sensitive pulse sequence creates an image of the tissue and the region being heated to allow the operator to adjust the position of the ultrasonic transducer so as to direct ultrasonic energy to the appropriate location.
U.S. Pat. No. 5,443,068 “Mechanical positioner for magnetic resonance guided ultrasound therapy” similarly discloses a non-magnetic positioning device of an ultrasound energy applicator, which is operated within the bore of a MRI magnet. The purpose of this invention is to provide a simplified positioner, which is operated within the magnetic filed. And also the interference to the magnetic field of MRI system due to the material of the positioning means is avoided. Other similar patents include U.S. Pat. No. 5,275,165 “Magnetic resonance guided ultrasound therapy system with inclined track to move transducers in a small vertical space” and etc.
The solutions described in the US patents as above mentioned are to place an ultrasound therapy transducer and positioning means within the static magnetic field of MRI system. The positioning means move the therapy transducer and the focal points of the transducer are used to expose the tumor and therefore the treatment on diseases and the monitoring in real-time have been achieved. The means to solve the technical problem is mainly aimed at the non-magnetic designs of therapy transducer and the positioning means and minimizing the volume of the positioning means and the operation space for it as far as possible to meet the working requirements of ultrasound energy applicator within MRI magnetic field. These technical solutions mainly have the following disadvantages: 1) Because the highly required non-magnetic designs and treatment on the therapy transducer and its positioning means, the technical complex and the cost are increased; 2) Because the positioning means are located within the magnetic field, the movement range of ultrasound transducer is limited strictly and furthermore the high-intensity focused ultrasound therapy needs a very precise locating system and the positioning means of the real therapy equipment in clinical applications are usually large, this technical solution increases the difficulties of equipment manufacturing and actual applications; 3) Because one part of the conductor used to supply power to the transducer is located within magnetic field, the magnetic field produced by the working current flow of the conductor will bring a big interference to MRI system, which is sensitive to magnetic signals; 4) It is difficult to perform assistant manual operations for an operator and also it is inconvenient for an operator to make clinical observations.
Chinese Pat. No. 98805359.4 “MRI guided therapeutic equipment and method” discloses a new type of MRI equipment. The static magnetic field (main magnetic field) of this MRI equipment is different from the closed or half-closed static magnetic field adopted by conventional MRI equipments. Its technical core is to provide a superconductive single-sided magnetic field. Therefore it is an open magnetic field in a big scale and it has some advantages for movement of therapeutic equipment and it can solve the problems encountered by the US patents as above mentioned.
But, the technical solutions for designs of this magnetic field still have some unsatisfactory points: 1) Weak magnetic field intensity and uneven magnetic line distribution influences the image quality. Under low field, the time for imaging is long and it is very difficult to realize the real-time imaging and the temperature measuring. The effective magnetic resonance volume, i.e. magnetic field available for magnetic resonance imaging is only several centimeters; 2) Because the more complex superconductive technology is needed to be applied, the complexity of the equipment is high and it is very difficult to put it in practice and it is a long time to be a mature applicable technology for this equipment.
Therefore, it is expected to provide a MRI guided ultrasound therapy system, which has a relatively low cost and is easy to be operated and particularly is suitable for high-intensity focused ultrasound therapy. Thereby, the ultrasound therapy technique can be further improved to enhance the safety and shorten the treatment time.